As a material for a cutting tool for use in a cutting work or the like, an aluminum oxide (Al2O3) sintered material is known. An aluminum oxide sintered material is excellent in that it has high hardness owing to very strong bonding strength between oxygen and aluminum, has low reactivity with an iron-based work material, and can be manufactured at low cost.
On the other hand, an aluminum oxide sintered material tends to have low toughness. This would be ascribable to the fact that the crystal structure of aluminum oxide is hexagonal, and has strong anisotropy, and the aluminum oxide sintered material is susceptible to transgranular fracture along the weak bonding direction such as a direction perpendicular to the c axis. Therefore, the aluminum oxide sintered material tends to be easily broken when it is used as a cutting tool.
Further, aluminum oxide has low reactivity with metal compounds such as nitrides and carbonitrides of metals in Group 4, Group 5, and Group 6 of the periodic table which serve as a binding material. Therefore, when a sintered material is produced by using aluminum oxide and these metal compounds, the obtained sintered material tends to be susceptible to chipping due to a defect or dropout of crystal grains.
Japanese Patent Laying-Open No. 2001-89242 (PTD 1) discloses, as a ceramic sintered member having high hardness, strength, and toughness in regions from the low temperature region to the high temperature region, and having high thermal shock resistance and thermal crack resistance, a ceramic sintered member made from aluminum nitride in place of aluminum oxide, and further containing a titanium compound.
Japanese Patent Laying-Open No. 4-300248 (PTD 2) discloses, as a ceramic sintered material suppressed in chipping due to a defect or dropout of crystal grains, a sintered material formed by sintering a mixed powder of 40 to 80% by volume of titanium nitride (TiN) and aluminum nitride (AlN) as a main component of a binder.
Aluminum nitride includes hexagonal aluminum nitride having a hexagonal crystal structure, and cubic aluminum nitride having a cubic crystal structure. Based on comparison of hardness between hexagonal aluminum nitride and cubic aluminum nitride, it is conceivable that cubic aluminum nitride having higher hardness is preferred as a material for a cutting tool. However, at normal temperature and normal pressure, hexagonal aluminum nitride is stable, and cubic aluminum nitride is metastable. Therefore, it is impossible to obtain cubic aluminum nitride at normal temperature and normal pressure.
Japanese Patent Laying-Open No. 2004-284851 (PTD 3) discloses a method of spraying a powder of hexagonal aluminum nitride and a carrier gas in an aerosol form to a substrate in a deposition chamber under reduced pressure, to change the crystal structure of hexagonal aluminum nitride to cubic aluminum nitride by the impact at the time of collision to the substrate.
“Proceedings of the Japan Academy”, 1990, Series B, Vol. 66, pp. 7-9 (Non PTD 1) discloses obtaining a powder containing cubic aluminum nitride and hexagonal aluminum nitride by treating hexagonal aluminum nitride under an ultra-high pressure and a temperature of 16.5 GPa and 1400 to 1600° C.
Also as a material for a cutting tool for use in a cutting work or the like, high speed steel (hereinafter, also referred to as HSS) is also used.
Japanese Patent Laying-Open No. 2002-16029 (PTD 4) discloses a method for coating HSS with an AlCrN hard film having excellent heat resistance and abrasion resistance.